MUSCATINE, Iowa—John Tompkins had voted for Barack Obama, but in 2016 he eagerly backed Donald Trump. “Trump is on the right track,” said the 83-year-old Air Force veteran. “He wants to get rid of the illegals.”
Mr. Tompkins’ friends, gathered for morning coffee at the Hy-Vee Grille, said they had made the same switch, motivated by Mr. Trump’s promises to boost military funding and stop illegal immigration.
Party-switchers like these helped propel Mr. Trump to the White House, in the process flipping 20 congressional districts into Mr. Trump’s column that had voted twice for Mr. Obama.
This year, a central question is whether these Obama-Trump voters will back Republican candidates with the same enthusiasm they felt for Mr. Trump, or whether the president’s unconventional appeal assembled a voter coalition that is unique to him.
Some analysts see the GOP challenges this year as similar to those Democrats faced in midterm elections when Mr. Obama was in the White House.
“The Obama coalition came out for Obama, but not when he wasn’t on the ballot. I think it’s the same for Trump,” said Chris Krueger, a strategist at Cowen & Co., a financial services and research firm. “They embraced the cult of personality around the candidate, not the party.”
The question is particularly pressing in places such as southeast Iowa, which makes up one of the 20 congressional districts that had backed Mr. Obama twice before supporting Mr. Trump. Seventeen of those party-switching districts are battlegrounds for the House this year, analysts say, giving them a central role in the battle between the parties for control of the House.
Nine of the districts are represented in the House by Republicans, and eight backed Democrats for the House while at the same time supporting Mr. Trump.
The political converts who backed Mr. Trump still support him enthusiastically, interviews in two of Iowa’s battleground districts show. “I like him being a rebel and I like the way he does things, truthfully,” said Jim Evans, a local real-estate investor. “It fires you up.”
But with Democrats showing high motivation in elections since Mr. Trump took office, House races in districts like this may turn on whether Republicans can match that apparent enthusiasm. Mr. Tompkins, for example, says he often doesn’t vote in midterm elections and isn’t sure he will cast a ballot this year.
In addition to motivating Trump supporters for the midterms, Republicans also have to hold voters like Tom Spread, the president of Muscatine’s Community Bank & Trust. He said he voted for Mr. Trump in 2016 only after great deliberation.
“The lesser of all evils was Trump,’’ Mr. Spread said. “Would I do it again? It depends on who’s on the ballot. I was holding my nose.”
His current inclination is to back a GOP candidate in the local House race who in the past drew distance from Mr. Trump. If a more Trump-supporting candidate wins the nomination, he said, he isn’t sure what he will do.
The Obama-Trump Districts
There are 20 House districts that voted twice for Barack Obama before flipping to back Donald Trump in 2016. These districts tend to be...

...and less educated.
Share of population with at least a bachelor’s degree
...older...
Median age of population
...less diverse...
Percentage of white, non-Hispanic population
Republican
representative
Democratic
representative
80
70
60%
90
40
38
44
36
42
20%
25
30
35
National rate, 61.3%
National rate, 37.9
National rate, 33.4%

...and less educated.
Share of population with at least a bachelor’s degree
...older...
Median age of population
...less diverse...
Percentage of white, non-Hispanic population
Republican
representative
Democratic
representative
90
60%
80
70
36
42
40
44
38
30
25
35
20%
National rate, 61.3%
National rate, 37.9
National rate, 33.4%

...and less educated.
Share of population with at least a bachelor’s degree
...older...
Median age of population
...less diverse...
Percentage of white, non-Hispanic population
Republican
representative
Democratic
representative
70
60%
80
90
42
38
40
44
36
35
20%
25
30
National rate, 61.3%
National rate, 37.9
National rate, 33.4%

...less diverse...
Percentage of white, non-Hispanic population
Republican
representative
Democratic
representative
80
70
60%
90
National rate, 61.3%
...older...
Median age of population
44
42
36
38
40
National rate, 37.9
...and less educated.
Share of population with at least a bachelor’s degree
35
20%
25
30
National rate, 33.4%
Source: Census Bureau (demographics); Associated Press (election results); U.S. House (members)
The twice-Obama, now-Trump districts tend to be more rural than the nation as a whole, largely middle income and not racially diverse. Many fall below the national average for share of residents with a bachelor’s degree.
They include three of Iowa’s four congressional districts, as well as those in other largely rural areas, such as western Wisconsin, upstate New York and in Minnesota. White voters with blue-collar jobs, who as a whole favored Mr. Trump by large margins in 2016, are plentiful in these districts.
All of those demographic markers are true for Iowa’s Second Congressional District, represented by Democrat Dave Loebsack. It is a sprawling mix of small towns and rural farmlands. For the most part, times are good here. The unemployment rate is below the national average, and there are plenty of manufacturers, including HNI Corp. , the nation’s second-largest maker of office furniture, and Kraft Heinz.
Still, many say times were better in the past. The city of Muscatine is trying to revitalize its downtown, which is dotted with vacant store fronts.
And while the district’s Hispanic population is still low by U.S. standards at 5.4%, the number is up from 3.4% in 2005. The growth has increased tensions, particularly with some older voters here, many of whom supported Mr. Trump in 2016, says Mary Wildermuth, executive director of the Muscatine History and Industry Center.
Ms. Wildermuth, a Republican, said she wrote in Florida GOP Sen. Marco Rubio’s name on her presidential ballot in 2016. She said she is unsure how she will vote this November.
In Iowa’s 1st Congressional District, just north of here, Republican Rep. Rod Blum will likely need to keep the party unified to win re-election. The seat was held by Democrats for four terms until 2015, and analysts list the race this year as a tossup.
Mr. Trump’s presidency has fired up Democrats in the city of Dubuque, where the past year has brought occasional protests outside Mr. Blum’s office, area businesses owners say. The 2016 election represented a seismic shift in the area. It was the first time since 1956 that the county gave its vote to a Republican presidential nominee.
Robert Felderman, a retired brigadier general who lives in Dubuque, said Mr. Trump won the county and the district due more to his personal appeal than to his party affiliation. Mr. Felderman, an independent voter who was born and raised in the city, said the political pendulum could swing again in November.
“The union guys are with Trump,” Mr. Felderman said. “They’re not Blum voters.”
Write to Dante Chinni at Dante.Chinni@wsj.com
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