WASHINGTON — Wisconsin’s Ron Johnson on Wednesday became the first Republican senator to say he opposes his party’s tax bill, signaling potential problems for GOP leaders. Passage of a similar package seemed certain Thursday in the House, where a handful of dissidents conceded they expected to be steamrolled by a GOP frantic to claim its first major legislative victory of the year.

Eager to act before opposition groups could sow doubts among the rank-and-file, Republican leaders were anxious to hand Donald Trump the first crowning bill of his presidency by Christmas. The two chambers’ plans would slash the 35 percent corporate tax rate to 20 percent, trim personal income tax rates and diminish some deductions and credits — while adding nearly $1.5 trillion to the coming decade’s federal deficits.

Republicans touted their legislation as providing tax breaks for millions of families and leaving companies with more money to produce more jobs.

“It represents a bold path forward that will allow us as a nation to break out of the slow-growth status quo once and for all,” said House Ways and Means Committee Chairman Kevin Brady, R-Texas.

Democrats said the measures would deliver the bulk of their benefits to higher earners and corporations. In the Senate Finance Committee, they focused their attacks on two provisions designed by Republicans to save money.

One would repeal President Barack Obama’s requirement that people buy health coverage or pay a fine, a move the nonpartisan Congressional Budget Office projects would result in 13 million more uninsured people by 2027. The other would end the personal income tax cuts in 2026 while keeping the corporate reductions permanent.

“We should be working together to find ways to cut taxes for hardworking middle-class families, not taking health care away from millions of people just to give huge tax cuts to the largest corporations,” said Sen. Bill Nelson, D-Fla.