McCaskill, saying Kavanaugh will boost the influence of wealthy donors in elections, became the first of the five most vulnerable Senate Democrats to oppose Trump's choice.
Brett Kavanaugh is the first Supreme Court pick opposed by a plurality of voters dating back to 2005, according to NBC/Wall Street Journal polls.
Sen. Dean Heller's vote for Obamacare repeal is perhaps the biggest issue in his re-election bid as President Donald Trump comes to Nevada to give the Republican incumbent a boost.
In a statement Wednesday, Collins said he would start to "actively campaign" after suspending his campaign in August.
Trump's boasts about future GOP success could damage his party in the midterms as it tries to stress the importance of voter turnout.
House Ways and Means Committee Chairman Rep. Kevin Brady, R-Texas, said "the time is short before the election" to approve Republican tax bills.
Democrats jumped on comments from White House economic advisor Larry Kudlow, who said the Trump administration will "probably" address "larger entitlements" next year.
Republicans had sounded the alarm after a series of polls showed a tighter race between Cruz and O'Rourke.
An ongoing and widening trade war poses political risks for Republicans as they try to stop Democrats from taking House and Senate majorities in November's midterms.
Commerce Secretary Wilbur Ross says new U.S. tariffs on China are aimed at modifying China's behavior.