A win for the Democrats in Congress next year may push U.S. President Donald Trump to resign rather than face a possible impeachment, says a political risk analyst.

“If the Democrats win in 2018 as far as the House of Representatives, they can then actually publish the tax returns of the president,” John Raines, head of political risk at IHS Markit, told CNBC’s Street Signs.

“So is there something in those tax returns that could actually cause damage to the president? All of a sudden he feels compelled to do something like resign.”

The president’s tax returns have been a hot button political issue since the election campaign, when Trump became the first major presidential candidate since the 1970s not to release his tax returns. The president claimed he could not release them because he was under audit by the IRS.

Raines says there would have to be a major issue within the tax returns for the majority of the Republican Party to turn against him.

“At this point, 80 to 85 percent of Republicans are still with this president. Republican congressmen, whether it be senators or house members, it’s going to be very difficult for them to make that (decision on impeachment) unless there’s some real smoking gun there.”

Last week, U.K. betting firm Paddy Power cut odds on Trump to resign from office, after Tony Schwartz, the ghost-writer behind the president’s memoir ‘Art of the Deal’, told CNN he predicts Trump will quit the job in months.

He also said on Twitter that he would be “amazed” if the president lasts until the end of the year.

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Bookmaker Paddy Power said large amounts of money was being bet on odds of 6/4 that Trump will resign, forcing the company to cut the odds to even money (where the profit on the bet will be the same as the amount staked).

“Tony Schwartz spent 18 months with Trump when helping ghost-write his memoir and while that must have been totally unbearable – it also means he knows The Donald pretty well,” Paddy Power said in a press release on Friday.

“After an awful week for the president that has seen other issues like North Korea pushed into the shadows it’s no surprise punters are latching onto the fact Trump might call it a day.”

Meanwhile, the president’s approval rating has fallen in the three key states of Michigan, Pennsylvania and Wisconsin, where Trump was the first Republican to win the presidential election since the 1980s, according to new NBC News/Marist polls.

More than six in ten voters told pollsters they were “embarrassed” by the president, according to the polls published on Sunday.

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