United States Sen. Charles Schumer, D-N.Y., has been a staunch supporter of President Barack Obama on most matters. But even Schumer has a limit.

Schumer revealed recently he will oppose Obama’s proposed pact with Iran over that aggressive country’s nuclear weapons program.

Obama insists the deal will prevent Iran from developing atomic bombs for 10 years. That is highly doubtful. In his eagerness to get an agreement – any deal he could claim means will promote peace – Obama and his chief representative at the negotiations, Secretary of State John Kerry, gave Tehran loopholes big enough to fire a nuclear missile through. Unfortunately, the Iranians may do just that.

Most Democrats are expected to support their party’s president on the issue. Some already have announced they will do so.

Schumer, the Democrats’ current No. 3 leader in the Senate, was and is under enormous pressure to back the pact. In announcing he will not do so, he acted with courage.

But that sort of fortitude should be expected when the issue is the security of Americans and Israelis.

Now that Schumer has taken the lead, other Democrats in the Senate should join him and the many Republicans who oppose the Iran deal. They should link arms firmly against it and demand Obama try again – and do better next time.