(Reuters) - Mitt Romney secured a close victory in Ohio but failed to land a knockout blow against rival Rick Santorum on Super Tuesday, raising the odds of a drawn-out battle for the Republican presidential nomination between the party's establishment and its grassroots conservatives.

Romney won five of the 10 state contests on Tuesday but his margin of victory was uncomfortably slim in Ohio, the night's biggest prize. Ohio is a politically divided state sure to be hotly contested in the November 6 election between the eventual Republican nominee and Democratic President Barack Obama.

Romney moved closer to the 1,144 delegates needed to win the party's nomination, but a strong showing by Santorum underscored the front-runner's continued inability to win over large swathes of the Republican base, who view his past as a moderate Massachusetts governor with suspicion.

We're counting up the delegates for the convention and it looks good, Romney told supporters in his home state of Massachusetts.

Romney won easily in Massachusetts, as well as liberal-leaning Vermont and in Idaho, where his fellow Mormons make up a substantial slice of the electorate. Romney also won Virginia, where Santorum failed to qualify for the ballot.

Santorum said his victories in Tennessee, Oklahoma and North Dakota proved he was the best candidate to represent the party's conservative philosophy.

Newt Gingrich won his home state of Georgia and said he would stay in the race.

With most of the votes counted in Ohio, Romney led Santorum by 38 percent to 37 percent and TV networks projected he would take the state. Romney ran strongest in and around the state's largest cities, while Santorum carried the rural areas.

Exit polls showed that Ohio voters viewed Romney as more likely to defeat Obama, but thought Santorum was more sympathetic to average Americans' concerns.

STRUGGLE TO CONNECT

Romney, who built a fortune of at least $200 million as a private-equity executive, has struggled to connect with conservatives and blue-collar voters. A convincing win in Ohio would have put many of those doubts to rest.

Romney extended his lead among delegates, not least because Santorum's thinly staffed campaign failed to qualify for delegates in several parts of Ohio. Under new rules designed to lengthen the nominating battle, most states at this stage of the process award delegates on a proportional basis.

Santorum, a former U.S. senator from Pennsylvania, has won support of religious conservatives thanks to his opposition to gay marriage and his views on other hot-button social issues. He also has targeted blue-collar voters, but his controversial comments about birth control and the role of religion have alienated moderate-leaning voters, especially younger women who will be a key constituency in November.

Only 21 percent of Ohio's voters on Tuesday were identified as women who work. But of those, Romney bested Santorum by a 7-point margin. And unmarried women of all ages favoured Romney by 16 points, 44 percent to 28 percent.

Romney and his allies outspent Santorum by nearly 4 to 1 in Ohio, according to campaign finance data, with much of that on negative ads.

We're going to get at least a couple of gold medals and a whole passel full of silver medals, Santorum told supporters. We've won in the West, the Midwest, in the South and we're going to win across this country. Gingrich's strategy of focusing on southern states did not pay off in Tennessee and Oklahoma, but he vowed to stay in the race after his Georgia win.

There are lots of bunny rabbits to run through, I am the tortoise. I just take one step at a time, Gingrich said.

The next big contests are in Kansas, Alabama and Mississippi, traditionally conservative states where Romney faces an uphill battle in the coming week.

Ron Paul, a U.S. representative from Texas known for his libertarian views, hopes to score his first win in Alaska.

In recent presidential campaigns, the Super Tuesday wave of primaries and caucuses has often settled the Republican race. But this year's race is likely to stretch until April or May - or possibly until the last contest on June 26 - under new rules designed to attract more voters and boost enthusiasm.

Recent polls indicate the lengthy primary season may actually be alienating voters as the candidates spend millions of dollars attacking each other and discussing issues like birth control that only resonate with the most conservative voters.