Dan Schwerin was director of speechwriting for Hillary Clinton's 2016 presidential campaign and collaborated on her memoir "What Happened."

As a Democrat, I have appreciated Republican senators such as Jeff Flake (Ariz.) and John McCain (Ariz.) warning about the damage President Trump is doing to our country. I wish they would oppose a tax bill that explodes the deficit and punishes working families. But if they're not going to do that, they should at least use this moment of maximum leverage to impose some checks on what they themselves describe as an out-of-control president.

Under normal circumstances, it would be surprising if GOP senators didn't fall in line. After all, no policy unites Republicans like cutting rich people's taxes. But these are not normal circumstances. Flake himself warned his Senate colleagues never to "regard as normal the regular and casual undermining of our democratic norms and ideals." Yet Flake, McCain, Susan Collins (R-Maine) and others gave away their votes on the tax bill for practically nothing. This was either terrible negotiating or a failure of imagination. Or maybe their big talk about Trump is just that — talk.

Trump's GOP critics should use their power as swing votes to extract specific concessions that would limit the damage the president can do. They missed a golden opportunity on Friday, but they will likely get another chance after the tax bill emerges from a joint Senate-House conference committee. Here are some concrete steps they should take.

First, they should protect special counsel Robert S. Mueller III's investigation. Now that former national security adviser Michael Flynn has pleaded guilty to lying to the FBI and is cooperating with Mueller, preventing Trump from rashly firing the special counsel is more important than ever. Two bipartisan bills have already been introduced to do just that, one by Sens. Lindsey O. Graham (R-S.C.) and Cory Booker (D-N.J.) and another by Thom Tillis (R-N.C.) and Christopher A. Coons (D-Del.). These lawmakers and their colleagues should insist that a version of their legislation be approved and signed into law before they vote for the final tax bill.

Second, the GOP anti-Trumpers should require some form of congressional approval for any future "first strike" attack with nuclear weapons. Even better would be a "no first use" policy for nuclear weapons. This wouldn't prevent the commander in chief from responding to an incoming attack on our homeland, but it would prevent the most terrifying danger we face under this president: the prospect of an angry and impulsive Trump firing off a nuclear missile the way he fires off a tweet.

Bob Corker (R-Tenn.), the chairman of the Senate Foreign Relations Committee who has warned that Trump is putting the nation "on the path to World War III," recently held a hearing on the issue. He and other Republicans concerned about national security and worried about Trump's capacity as commander in chief should strongly consider something like Sen. Edward J. Markey's (D-Mass.) legislation, which would put a check on this doomsday nuclear scenario.

Third, they should force the president to meet basic standards of ethics and transparency. The House GOP has voted down legislation that would require Trump to release his tax returns and official visitor logs from the White House and Mar-a-Lago. Flake and like-minded Republicans should make passing such legislation a prerequisite for final passage of the tax bill.

This list is just the beginning of what Congress could do to constrain the risks of the Trump presidency. Of course, the best outcome would be for Republicans in both houses of Congress to vote the final tax bill down on the merits. But if that's not going to happen, then the president's critics should insist on these steps.

Many Republicans won't want to vote for these measures and Trump certainly won't want to sign them. But they really want to cut taxes for billionaires. Now may be the single best opportunity to force real action. Based on the dire warnings coming from Flake and Corker, we can't afford to wait another minute.