Senate Majority Leader Mitch McConnell at a press conference in December. Mark Wilson/Getty Images On Jan. 20, Donald Trump became the 45th president of the United States. For Trump (as with every president before him), all that he said up until that point consisted of promises.

But now, the focus must shift... from what he will do, to what he is doing. (Subscribe to my free weekly newsletter, This Week in Geopolitics, and stay informed on the most important geopolitical developments)

Presidents don’t stop promising, but the promises become more hollow over time if they are not matched by some degree of achievement.

The president’s weak position

The American presidency is a paradox.

It is the most noted position in the world. Observers imbue it with the same level of command held by the world's most powerful country. That’s why everyone is now trying to understand what Trump intends to do.

In reality, the American president is one of the weakest institutional leaders in Euro-American civilization. He can do some things unilaterally, particularly in foreign policy, but Congress can block them.

He can do some things by executive order, but the Supreme Court can overrule them. He can pass certain programs that require cooperation from states, but the states can refuse to do so.

At every step, as the founders intended, his ability to act unilaterally is severely limited.

What will Congress do?

So now, the most important question is not, what does Trump intend to do… but instead, what will Congress do? Both chambers have Republican majorities. Republican control of the House of Representatives is overwhelming. Republican control of the Senate, though, is not.

The Senate has 52 Republicans, 46 Democrats, and two independents who are likely to vote with the Democrats. This, in essence, gives the Republicans a four-vote majority. Because the vice president would be the deciding vote in a tie… and because he is a Republican, three Republicans would have to switch sides to defeat any legislation.

Under the Constitution, senators are not elected to rubber-stamp the president. They represent their states. So this battle will not be between Republicans and Democrats. Nor will it be between both chambers. The real battle will be among Senate Republicans.

Three defections make it impossible to pass any proposed legislation. As such, any Republican senator who can position himself as a potential defector will be able to negotiate for the president’s support on a number of issues. The president will either be forced to compromise or risk having the legislation defeated.

Trump’s approval ratings are key

Senators are not free actors. They need to be re-elected. Their decision to oppose a Republican president will depend heavily (if not entirely) on whether the president will help or hurt their re-election bids. And that depends on the president’s approval ratings… particularly in the senators’ home states.

According to a Fox News poll taken just before Inauguration Day, 37% of those polled approved of Trump’s performance and 54% did not. Therein lies Trump’s problem… and battleground.

President George W. Bush, President Richard Nixon, President Lyndon B. Johnson, and President Harry S. Truman all had approval ratings around 37% toward the end of their terms. This number is normal for a failed or worn-out presidency.

I know of no president in the 20th century who began his term this way. Each party historically commands about 40% support among voters. When a president falls below 40%, he is actually losing support from his own party. It is normally hard to come back from that… and it usually takes years to get to that low level.

This poses a problem for Trump’s administration. With these numbers, it’s possible that more than three Republican senators could decide that rigid support for the president might cost them their political lives.

Trump’s approval ratings are not likely to fall below 37%. But to be effective, he can’t stay at that level. Republican senators will look at Trump’s negative ratings in their states and calculate whether supporting his programs might lock 50% of voters against them.

Because public support wanes over the course of a presidency (though it sometimes blooms with nostalgia later in his term), it is essential to start a term with as much support as possible. If Trump wants to get his controversial bills passed, he must build his popularity quickly.

His staff, particularly the vice president, will be examining every Republican senator who is up for re-election in 2018 to determine how to help sway their states’ voters. Trump’s fear will be that he will alienate his core while failing to make inroads with his enemies.

The power of the filibuster

The final point to consider is the use of filibusters. This is a deep tradition in the Senate, and it has served as another check on power that the founders would have been proud of. Any senator may filibuster a bill. If a whole party does it, the filibuster can only be stopped by getting 60 votes in favor or by letting the senators go on until they drop.

If they act as a party, the Democrats in the Senate would be able to block Trump’s entire agenda. Alternatively, Trump would need the support of eight Democrats to get 60 votes to end a filibuster. That isn’t likely to happen.

The president can achieve some things with an executive order, assuming the Supreme Court doesn’t step in. But broader policies like infrastructure development won’t get passed without congressional support.

The battleground will be within the Republican Party in the Senate. The result will depend on whether Trump’s approval ratings increase above 37%. Just holding there won’t do it. That number has been “Death Valley” for other presidencies… although this is a unique case since we have no way to benchmark a presidency that starts at this level.

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