President Donald Trump invited a slew of senators to join him at the White House Monday night when he unveiled his second nominee for the Supreme Court, federal appeals court Judge Brett Kavanaugh.

But three Democrats who voted for Trump's last Supreme Court pick, Justice Neil Gorsuch, all turned him down this time.

Sen. Joe Manchin, D-W.Va., said he appreciated Trump's invitation, but instead wanted to meet the president's nominee "in a setting where we can discuss his or her experience, judicial philosophy & perspective" on political issues.

Manchin tweet

Indiana Democrat Sen. Joe Donnelly declined for similar reasons. "I declined so that I can meet first with the nominee in a setting where we can discuss his or her experience and perspectives," Donnelly said in a statement.

And a spokeswoman for Sen. Heidi Heitkamp, D-N.D., said Heitkamp "isn't able to attend tonight" because "she considers fully vetting Supreme Court nominees one of the most important jobs of any U.S. senator, and she plans to fulfill that critical duty."

All three Democratic senators voted for Gorsuch. And all of them could soon be targeted by pro-Trump political action groups trying to unseat Democrats in the states Trump won in the 2016 presidential election.

The White House did not immediately respond to CNBC's inquiries.

Even though Republicans hold a majority in the Senate, their razor-thin 50-49 margin (assuming the ailing Sen. John McCain, R-Ariz., does not vote) leaves open the possibility that Democrats could block Trump's nominee.

If Democrats can maintain total opposition to Trump's pick — and if they can get a single Republican to break ranks and vote against the nominee — they could stop Trump from cementing the Supreme Court with an enduring conservative majority.

But some Democratic senators — including Manchin, Donnelly and Heitkamp — are sure to face immense pressure to vote for Kavanaugh.

They're not the only Democrats to turn down Trump's invite. Democratic Sen. Doug Jones, who won a shocking special election in deep-red Alabama against Roy Moore, declined to attend the event though he has said he would be open to voting for Trump's forthcoming nominee.

Others weren't invited at all. Sen. Jon Tester, D-Mont., has been a regular target of Trump's ire for months since he spoke out about allegations against Ronny Jackson, Trump's former White House physician. Jackson dropped out of his nomination to lead the Veterans Affairs Department soon after — an ending that led Trump to call for Tester's resignation.

Last week, the president tore into Tester during a rally in the senator's own state, and encouraged the audience to vote him out of office.

A spokeswoman for Tester confirmed that the red-state Democrat had not been invited to the White House.

But some GOP senators are following the Democrats' lead: Sens. Susan Collins of Maine and Lisa Murkowski of Alaska.

Collins and Murkowski are widely viewed as the two Republicans most likely to vote against a nominee they find objectionable. Collins, one of the few pro-choice Republicans in the Senate, has said her support hinges on a nominee's support for Roe v. Wade, the controversial decision that legalized abortion in the U.S.

A spokeswoman for Collins said "she appreciates the invitation, but will not be attending." Murkowski declined the president's invite, her spokeswoman said, just as she had declined the announcement of then-nominee Neil Gorsuch.