The historic nuclear deal with Iran marks the first time in three decades that the Persian nation has agreed to slow its work toward a nuclear weapon and allow international monitors in to verify.

It's a significant accomplishment, but the accord is about to become entangled in U.S. politics for months to come, complicating the pact's future on both sides of the Atlantic.

Here are five reasons why:

1. President Obama's Credibility

Obama's domestic political difficulties came into play practically as soon as the deal was announced. It takes public trust for a president to sell an agreement like the one with Iran. Such trust was once among Obama's main strengths. His credibility, however, was damaged by the botched Affordable Care Act rollout and his overselling of some aspects of the law.

Now, judging by recent polling, he faces more skeptical voters. His political opponents and those opposing the deal — frequently the same people — are already using this against him.

Take Sen. John Cornyn, R-Texas, who on Sunday tweeted, "Amazing what the WH will do to distract attention from O-care," and retweeted musician Charlie Daniels: "If you like your nuclear [program], you can keep your nuclear program."

2. Election-Year Politics

While Obama doesn't face a 2014 re-election — or re-election ever again, for that matter — all of the House and a third of Senate seats are on the ballot next year. The timing of the six-month Iranian nuclear deal means it will coincide with party primaries. And a follow-up deal, if there is one, would coincide with the November midterm elections. That timing guarantees the agreement will become part of superheated congressional campaigns.

3. Israel

U.S. Middle East policy involving Israel almost always blurs the normal political battle lines, uniting some Republican and Democratic Israel hawks who seldom agree on anything else.

This deal is no different. The points of agreement are: Iran gets more than it's giving up; the agreement takes pressure off Iran; and Iran is simply playing for time and still intends to get nuclear weapons. Thus, Sen. Charles Schumer, D-N.Y., who seldom parts company with Obama, sounds a lot like Sen. Ted Cruz, R-Texas, who seldom agrees with the president.

Meanwhile, opposition to the agreement could certainly fire up some voter segments, like Christian evangelicals in the GOP who tend to side with Israeli conservatives. And enforcing the anti-agreement line will be AIPAC, an influential pro-Israel lobby that puts little daylight between itself and the Israeli government led by Prime Minister Benjamin Netanyahu.

4. The Prospect Of Additional Sanctions

There's still interest in exploring additional sanctions against the Iranian regime, despite the Obama administration's request for congressional forbearance. Such additional sanctions, rooted in deep skepticism over Iran's intentions, could scuttle the deal.

5. The Legacy Agenda

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