President Obama wants Republicans to agree to tax rate increases and new stimulus spending now in return for entitlement and tax code reform in the future. Accepting anything resembling that bargain would be bad politics given the fiscal principles on which Republicans have stood for the last two years. It would also suggest that Republican members of Congress need a few lessons in the fundamentals of constitutional authority and popular sovereignty.

Agreements of the sort urged by the president are not uncommon. The context is usually something like this. There is a big problem — say, a burgeoning federal debt — that cannot be solved overnight. There is no action Congress can take today that will eliminate the national debt or ensure that Social Security and Medicare will be solvent 10 or 20 years from now. Problems that have developed over decades require some time to correct.

The constitutional reality is that Congress and the president will have to chip away at these big problems and hope that those who follow in the White House and Congress will do the same. There is no way of guaranteeing that future presidents and Congresses will not abandon measures implemented today. But unless responsible steps are taken now, we’ll have little hope of meeting these long-term challenges in the future.

Politics being what it is, though, the current president and Congress would much rather do the easy stuff now and leave the more painful actions to those who come later.

Republicans shouldn’t let that happen.

Aside from the obvious danger that the president will either go back on his word or fail to persuade congressional Democrats to implement what he has promised, there are constitutional reasons that such agreements are unenforceable and thus a sucker’s bargain.

First, despite the implications of President Obama’s “we can’t wait” approach to executive power, and despite the repeated claims by candidates Obama and Romney that if elected they would enact and repeal particular laws, the president lacks any such constitutional authority. The president can no more guarantee future legislative action than I can guarantee that Warren Buffett will write you into his will at some future date.

Second, the current Congress cannot bind or constrain a future Congress. So even if Harry Reid and Nancy Pelosi join President Obama in promising to deliver future entitlement and tax code reform in return for current tax rate increases and new stimulus spending, their promises will have no legal effect. Even if those promises are written into a law enacted by Congress and signed by the president, they won’t bind a future Congress.

This principle was set forth long ago by English jurist William Blackstone and quoted in a Supreme Court opinion by Justice David Souter as recently as 1996: “The legislature, being in truth the sovereign power, is always of equal, always of absolute authority: it acknowledges no superior upon earth, which the prior legislature must have been, if it’s [sic] ordinances could bind the present parliament.” In other words, if a future Congress is obliged to enact entitlement and tax code reforms promised by today’s Congress, then the future Congress is not sovereign and the people who elect it are not the source of all government authority as proclaimed in the Declaration of Independence.

Recognizing that there is no constitutional way to bind future presidents or Congresses (except by constitutional amendment), Republicans should be wary of promised future actions. Trading in political futures is a fool’s bargain. Best to give up no more than you get in present-day, binding agreements.

Of course, an irony in the fiscal cliff negotiations is that the most affected parties are not even at the bargaining table. That’s why it’s easy for both sides to kick the can down the road. Our children and grandchildren will pay the piper if the president and Congress do not start solving today’s problems today. Forget about the top 2%. Representing the interests of all future taxpayers is a pretty good principle for Republicans to stand firm on.

Jim Huffman is the dean emeritus of Lewis & Clark Law School, the co-founder of Northwest Free Press and a member of the Hoover Institution’s De Nault Task Force on Property Rights, Freedom and Prosperity.