Trump, who called on McCaskill to vote for the tax plan earlier this year, lent his support to Missouri Attorney General Josh Hawley's bid to unseat the vulnerable Democratic lawmaker, since McCaskill has refused to come on board.

President Donald Trump's visit Wednesday to Missouri to promote a GOP tax plan may be less about winning Democratic Sen. Claire McCaskill's support than boosting her opponent.

Trump, who called on McCaskill to vote for the tax plan earlier this year, lent his support to Missouri Attorney General Josh Hawley's bid to unseat the vulnerable Democratic lawmaker, since McCaskill has refused to come on board.

Trump recognized Hawley from the stage as he returned to Missouri to try to push the tax plan across the finish line.

"He's going to be a great senator," Trump said, adding that Hawley "wants to see a major tax cut" while McCaskill "wants your taxes to go up."

"I said, 'Josh when you're ready, you have my word, I'm going to come here and campaign with you,'" Trump said.

McCaskill, a top Republican target in the 2018 midterm elections, voted against the GOP proposal in committee earlier this month, and is expected to vote against it when the full Senate takes up the measure.

In his speech in the St. Louis suburbs, Trump made a populist appeal for the Republican tax package, saying it would "bring Main Street rolling back."

"A vote to cut taxes is a vote to put America first again," Trump said in St. Charles.

But while the White House says the plan will be a boon to middle-income families by helping small-business owners and workers, sparking economic growth and simplifying the tax code, critics say both the House and Senate versions will disproportionately help the wealthy and corporations.

The visit marks Trump's second to the state to sell his plan. During his trip to Springfield in August, Trump pledged the plan would "bring back Main Street."

The trip comes a day after the Senate Finance Committee advanced a sweeping tax package to the full Senate, handing Republican leaders a victory as they try to pass the nation's first tax overhaul in 31 years. But the bill still faces hurdles in the Senate, where Republicans have just two votes to spare in their 52-48 edge over Democrats.

McCaskill is among 10 Senate Democrats up for re-election in 2018 in states won by Trump and is considered one of the most vulnerable incumbents. She was among 45 Senate Democrats who in August sent a letter to Republican leaders and Trump saying they won't support any GOP effort to overhaul the tax system that delivers cuts to the top 1 percent or adds to the government's $20 trillion debt.

Trump said in August that if McCaskill didn't support his plan, voters should push her out of office.

"We must lower our taxes, and your senator, Claire McCaskill, she must do this for you. And if she doesn't do it for you, you have got to vote her out of office," Trump said then.

Several hundred Trump opponents and about 100 Trump supporters gathered outside the St. Charles convention center as the president spoke, holding signs and chanting competing slogans.