If you’re waiting for congressional Republicans to oppose any of President Donald Trump’s policies, you could be waiting a long time. Even if they were critical of him during the 2016 campaign, the only Republicans likely to take on the president now are those who aren’t up for re-election in 2018.

Trump’s policies are popular among the GOP base ― which means Republican lawmakers who speak out against the president could end up damaging their own chances in next year’s primaries.

For the most part, members of Congress behave like “single-minded seekers of re-election.” They advertise the good they’re doing, take credit for things that are going right and take positions that are popular.

David Mayhew, a Yale political scientist, laid out this idea in a 1974 book. It rings painfully true in 2017 ― except that “popular” means something different now than it did 40 years ago. In Mayhew’s time, policy positions needed to be popular with the entire electorate. In today’s polarized environment, most members of Congress only need to appease their party’s base.

Most Congressional seats are “safe” these days ― that is, likely to stay in the hands of whichever party currently has them. Relatively few seats are realistically up for grabs. Most of the Republican senators up for re-election in 2018 are in safe Republican states, with the exception of Jeff Flake in Arizona and Dean Heller in Nevada. That means the main threat to most sitting members of Congress is a primary election challenge from within their own party.

Congressional Republicans know all too well about the threat of primary challenges when the base is unhappy. Although defeating an incumbent in a primary doesn’t happen often, Republican Sens. Bob Bennett (Utah) and Lisa Murkowski (Alaska) were defeated by tea party challengers in 2010. Murkowski launched a successful write-in campaign and retained her seat; Bennett did not. A handful of Republican House members have faced a similar fate, notably former Rep. Eric Cantor (R-Va.), whom Dave Brat successfully primaried out of a job in 2014.

To avoid this type of challenge, congressional Republicans need to stick with what’s popular with the base. And right now, the base still likes Trump and his policies.

Even though most Americans disapprove of the job Trump is doing in office, an average of 89 percent of Republicans nationally approve. Only 10 percent within the party disapprove. For comparison, Barack Obama’s approval rating among Democrats was 88 percent in mid-February of his first term as president. His disapproval rating among Democrats was only at 3 percent at that point, but a few months later his approval was static and his disapproval increased to around 10 percent. So Trump is in good shape within his own party for now.

Many of Trump’s actions in the first three weeks of his presidency have been controversial among the U.S. population as a whole ― but not among Republicans. According to a HuffPost/YouGov poll, 88 percent of Republicans approve of the executive order barring travel from seven Muslim-majority countries for 90 days and suspending refugee entry for 120 days. Those numbers vary a bit depending on how the poll question is asked, but Republicans are overwhelmingly supportive of the policy in every implementation.

In perhaps the biggest red flag for vulnerable lawmakers, a January HuffPost/YouGov survey showed that 52 percent of Trump voters say they’d side with the president over Republicans in Congress. Forty-nine percent of all Republicans said the same, with only 15 percent saying they’d side with congressional Republicans.

In a more recent Quinnipiac University poll, 87 percent of Republicans said they want their elected officials to get along with Trump. Only 7 percent said their elected officials should stand up to the president.

If public opinion among Republicans shifts against Trump, the situation for congressional Republicans will change. But as long as base Republicans support the president, there’s no real incentive for GOP lawmakers to challenge him on anything.