In December, the Democratic congressman Jeff Van Drew, facing a tough re-election campaign in New Jersey, defected to the Republican party. On Tuesday, in return for his fealty, Van Drew was rewarded with an appearance at a Donald Trump campaign rally in his home state.

For Democrats contemplating their own switch before 2020, however, the evening revealed the cost of such a pact with Trump.

Van Drew has acquired some of Trump’s supporters, and managed to draw the eye of the nation’s media to his congressional district. But in exchange, Van Drew has been forced to give public, full-throated praise for Trump, and required to loudly espouse the nationalistic “America first” rhetoric his former colleagues fear and despise.

Thousands of Trump supporters had spent hours on Tuesday shivering outside a convention center in Wildwood, a tired seaside resort on the south-eastern tip of New Jersey. A line of supporters, swaddled in sleeping bags and blankets and plonked into camping chairs, snaked back from the boardwalk. An illuminated ferris wheel and a rollercoaster promising an “adrenaline rush!” loomed in the distance.

Trump knows New Jersey well enough – he bankrupted three casinos here in the 1990s – but this was his first campaign event in the reliably Democratic state since becoming president.

It was a real coup for Van Drew, in his first term in the House of Representatives. The 66-year-old was a Democratic politician in New Jersey for almost 20 years before he jumped ship in December.

Van Drew’s congressional district voted for Trump in 2016, and the Republican party in New Jersey had conjured a strong candidate to challenge him in the November 2020 election. Faced with being booted out of Congress, Van Drew jumped.

In a choreographed meeting at the White House after the loyalty switch, Van Drew said Trump had his “undying support”, always. Trump, in turn, said he endorsed Van Drew. The benefits became apparent almost immediately, when the wealthy Republican planning to run against Van Drew instead switched his focus to a neighboring district. But the defection has left a sour taste in the mouth of New Jersey Democrats.

“Betrayed, disappointed,” was how Betty Keller, 73, described her feelings toward Van Drew. “He was a lovely man, but he’s sold his soul.”

Keller was standing outside the center with her husband, Bill Keller. They weren’t there to see Trump. They were conducting a mild-mannered, low-key protest, which involved holding a cardboard sign that said: “Remove.”

“He copped out because locally he was going to get decimated by the Republicans anyway,” Bill Keller said. “His future was gone if he didn’t go with Trump. That’s what he thought.”

Bill Keller, a 71-year-old Vietnam war veteran, said some people had expressed “gratitude” to the couple for their protest. The Guardian didn’t see any evidence of this, but did witness a man in a red baseball cap take umbrage with the Kellers’ sign.

“Remove who? Remove Trump? You don’t like it, pack your shit and get out,” the man said.

Inside the rally, Trump avoided mentioning the impeachment trial – minutes before he went on stage it emerged Republicans may not have the votes to block witnesses from appearing before the Senate, which could stretch the trial for weeks – and instead praised Van Drew as “a courageous leader”.

“Jeff had the guts to defy the leftwing fanatics in his own party and to stand tall,” Trump said. “He’s a great guy. He’s a brave guy. He shares our values.”

A relatively conservative Democrat, Van Drew does share some values. He has a 100% rating from the National Rifle Association, and as a New Jersey state senator voted against legalizing same-sex marriage, and against raising the state minimum wage. Van Drew was also opposed to impeaching Trump.

The Van Drew case reveals the decision some other politicians face, too.

Do they embrace Trump and accept the benefits, or attempt to take a stand against the president, and risk losing their seat? Justin Amash, a Republican congressman from Michigan, decided on the latter when he ditched the GOP in July to become an independent. Amash’s chances of winning re-election are currently rated as a “toss-up” by the polling expert Larry Sabato.

It has been less than two months since Van Drew switched sides, but on Tuesday he showed he has quickly adapted to the Republicans’ strategy of flattering Trump at every turn.

“Our president has made it clear by restoring our military, protecting our economy and by not allowing any other nation to take advantage of us again. Do we want to keep it that way?” Van Drew asked the crowd.

Adopting Trump’s own rhetoric, Van Drew said the US is “a great nation, the greatest nation that civilization has ever known”.

As the crowd cheered, Van Drew asked: “Are we going to allow ourselves just to be like any other nation in the world or are we going to keep America great?”

It prompted a chant, not for the first time of the night, of “USA! USA! USA!”

But Van Drew may find himself more flustered when he is forced to confront New Jersey voters disgusted by his volte-face.

“There are a lot of good people who put quite a bit of money and effort into electing him on the premise of who he was and he misrepresented himself,” said Mike Freihofer, who was standing outside the rally with a sign that said “No kids in cages”, a protest at Trump’s hardline immigration policies, including across the US-Mexico border.

Friehofer, 54, voted for Van Drew in 2018. Like Betty Keller, he said he felt “betrayed” by Van Drew.

“It was a very dishonorable thing to do. At the very least.”