Barack Obama conceded yesterday that US voters were nervous about making him their next president as fresh polls showed him in an increasingly tight race against John McCain, his Republican rival.

The Democratic candidate sought to explain why he has not seen a significant bounce in the polls after his international tour last week - with a new survey showing Mr McCain taking a lead for the first time since Mr Obama secured his party's nomination.

Mr Obama's aides say that it is relatively early in the general election cycle, but there is a growing anxiousness about why he is not doing better against Mr McCain, who has so far run an unimpressive, disjointed and at times shambolic campaign.

The Democrat said that voters were still sizing him up and that his candidacy was “new for them, new for us as a country.

“This is going to be a close election for a long time because I'm new on the national scene and people sort of like what they see but they're still not sure.” But he added: “The odds of us winning are very good.”

Most recent surveys show Mr Obama about six points ahead, but stuck several points below the 50 per cent threshold. A USA Today/Gallup poll yesterday showed the Republican four points ahead - 49 per cent to 45 per cent - among likely voters, in the first poll taken since the Democrat's overseas tour. It showed a surge since last month among likely Republican voters, suggesting that the trip might have galvanised them.

What concerns Mr Obama's supporters is that by every measure he should be doing much better. In generic polls, voters overwhelmingly want a Democrat in the White House next year and a record number believe that the country is on the wrong track.

The Illinois senator is running a sharp, disciplined campaign - often setting the day's agenda - with Mr McCain appearing slow-footed and reactive.

Yet in recent polls the message is clear: voters may want change, but they are uneasy about Mr Obama. Both campaigns admit that the election is becoming a referendum on Mr Obama, testing the willingness of voters to overcome their doubts about a 46-year-old African-American with little political experience, to whom many find it hard to relate.

In a Wall Street Journal/NBC poll last week, half of those questioned said that they were focused on what sort of president Mr Obama would be, with just a quarter focused on what kind of leader Mr McCain would be. Asked who was the riskier choice, 55per cent said Mr Obama, to just 35per cent who said the same of his rival.

In a private conference call with supporters last week, Steve Schmidt, Mr McCain's chief strategist, vowed to sharpen attacks on Mr Obama and to try to increase the perception that he is a risk.

Mr McCain, who at 71 is the oldest US presidential candidate in history, said last week: “They need a steady hand on the tiller. That's what I'm going to convince them of.”

He has also begun to attack Mr Obama's patriotism. In a new advertisement the Republican campaign pounced on Mr Obama's decision to call off a visit to wounded US troops in Germany. “He made time to go to the gym, but cancelled a visit with wounded troops. Seems the Pentagon wouldn't allow him to bring cameras,” the narrator says.

The Obama camp countered that the visit had been scrapped amid concerns that it would appear too political.

Nagging at Mr Obama are memories of his big defeats to Hillary Clinton in the later stages of their primary battle, when white, working-class voters turned their backs on him.

Going back farther, there are other nail-biting historical precedents: in three elections when there was no incumbent president on the ballot - 2000 (Bush-Kerry), 1968 (Nixon-Humphrey) and 1960 (Nixon-Kennedy) - the winner entered the White House with a margin of victory of less than 1per cent.

History lessons

1980 Mr Obama's aides believe that this election is similar to the contest when voters had become disenchanted with President Carter but were unsure about Ronald Reagan. Only when Reagan quelled those doubts with an impressive performance in their one debate late in the campaign did support for Carter collapse. Reagan won by a landslide

1948 The McCain team hope to emulate Harry Truman, whose chances of winning in 1948 against the young, eloquent Thomas Dewey were dismissed by press and pundits. On election night the Chicago Daily Tribune even printed “Dewey defeats Truman”. Truman won with 49 per cent of vote to Dewey's 45

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