President Obama hit the crucial 50 percent threshold in the latest poll of Ohio and now leads Mitt Romney by 4 points in the pivotal battleground state, according to a new poll released Friday by CNN.

The slight but significant lead mirrors other recent polling in the state, and should come as a relief to the president's campaign, which has largely staked its reelection hopes on holding the Buckeye State. The poll carried a margin of error of 3.5 percent, just below the president's lead.

The president was boosted in the state by a significant lead among women, leading Romney 56 to 42 percent. Romney posted a smaller advantage among men, leading the president 50 to 44 percent.

The opportunity for much movement at the polls could be narrowing for both campaigns. Of the likely voters surveyed, 92 percent said they had already made up their mind about who they would vote for.

Ohio's 18 electoral votes are considered must-win by both campaigns. No Republican president has ever won the White House without capturing Ohio, and Obama would likely need a surprise win in Florida or Virginia to offset a Romney victory there.

On Friday, Sen. Rob PortmanRobert (Rob) Jones PortmanMcConnell names Senate GOP tax conferees Overnight Finance: House approves motion to go to tax conference — with drama | GOP leaders to consider Dec. 30 spending bill | Justices skeptical of ban on sports betting | Mulvaney won't fire official who sued him How four GOP senators guided a tax-bill victory behind the scenes MORE (R-Ohio), a co-chair of Romney's presidential campaign, told NBC News that it would be tough to see the Republican nominee winning the race without capturing Ohio.

"I'm feeling the pressure not just because I'm chairing the effort here in Ohio, but mostly because I feel the pressure for our country and what's going to happen over the next four years," Portman said. "If we don't win Ohio, it's tough to see us winning the election nationally. It's possible, but it's very difficult."

Romney criss-crossed the state Thursday, attending three campaign rallies, and will return there Friday night for a rally with running mate Paul RyanPaul Davis RyanMcConnell names Senate GOP tax conferees House Republican: 'I worry about both sides' of the aisle on DACA Overnight Health Care: 3.6M signed up for ObamaCare in first month | Ryan pledges 'entitlement reform' next year | Dems push for more money to fight opioids MORE. Obama visited the state on Tuesday with Vice President Biden.