Sen. Claire McCaskill already was anticipating a tough fight from Republicans as she seeks a third term, but her path got even trickier this month when she found out she now faces a challenge from her left as well.

Ms. McCaskill is perhaps the most vulnerable Democrat in the Senate heading into next year, where she will try to defend her seat in Missouri, a state that’s tilted heavily toward Republicans in recent years.

The National Republican Senatorial Committee has been showering Ms. McCaskill with criticism for months, accusing her of joining fellow Democrats in obstructing President Trump’s agenda.

Another GOP-aligned group, Missouri Rising Action, has formed a political action committee intended to entice Missouri Attorney General Josh Hawley to jump into the race to challenge the two-term senator.

The left has its own complaints about Ms. McCaskill, and 31-year-old Angelica Earl jumped into the race this month, vowing to mount a primary challenge while representing the Bernard Sanders progressive wing of the Democratic Party, with a push for single-payer health care, looser marijuana laws and compassion training for police officers.

“McCaskill probably is the most vulnerable Senate Democrat both because of Missouri’s turn to the right over the past decade or so and a decent chance that she’ll have a strong challenger, likely state Attorney General Josh Hawley,” said Kyle Kondik of the University of Virginia’s Center for Politics.

Nonpartisan political handicappers view the contest as a tossup right now, and say Ms. McCaskill has several things going for her.

She’s avoided several major challengers on both sides of the aisle. Rep. Ann Wagner, a Republican, took a pass on the race, as did former Secretary of State Jason Kander, a Democrat who has emerged a favorite of progressives after nearly upsetting GOP Sen. Roy Blunt in 2016.

“As Jason has said numerous times, he is going to do everything he can to help his good friend Claire McCaskill get re-elected in 2018,” said Austin Laufersweiler, a spokesman for Mr. Kander’s new venture, Let America Vote, an effort to push back against President Trump’s claims of voter fraud.

Should Ms. McCaskill survive the primary, analysts said she’ll benefit in the general election from historical trends that make midterm elections a referendum on the sitting president.

“McCaskill should not be underestimated,” Mr. Kondik said. “If [Hillary] Clinton was in the White House, she’d probably be in deep trouble. But with Trump in the White House, the party out of the White House gets a boost historically.”

The NRSC released a new rodeo-themed radio ad last week trying to paint Ms. McCaskill as being out of touch with the Missouri voters who handed Mr. Trump an 18-point win in the state last year.

“She says she is willing to work with Republicans, but she has fought President Trump all the way,” the narrator says in the ad. “McCaskill was all-in for Hillary, called her heads above the field. OK, well, election is over, your horse lost, time to move on, and since Trump became president, McCaskill has fought him tooth and nail.”

The McCaskill campaign did not respond to an email seeking comment.

Meira Bernstein, a spokesman for the Missouri Democratic Party, dismissed the attacks, saying Ms. McCaskill has put her constituents first, worked with Mr. Trump on legislation and voted to confirm more than half of the president’s campaign nominees.

“National Republicans’ attempt to claim otherwise is just smoke and mirrors from a party that is pinning its hope of defeating a strong, Missouri-first senator on a candidate who broke his promise to Missourians not to be another ladder-climbing politician just eight months after taking office,” Ms. Berstein said.

A FiveThirtyEight analysis shows that Ms. McCaskill has voted 42.6 percent of the time in line with the Trump agenda.

She voted against the Trump administration’s push for an Obamacare repeal, the confirmation of Neil M. Gorsuch to the U.S. Supreme Court and Jeff Sessions to be attorney general. She supported the White House’s push to repeal a stream protection rule, Rick Perry’s nomination to be energy secretary and Ryan Zinke’s nomination as interior secretary.

In addition to Mr. Hawley, several other Republicans are considering running for the nomination: former Missouri GOP chief Ed Martin, state Rep. Paul Curtman, state Treasurer Eric Schmitt and state Rep. Vicky Hartzler.

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