Barack Obama and John McCain both swept to victory last night in three US presidential primaries.
The surging Illinois Senator coasted to wins in Virginia, Maryland and Washington DC, while presumptive Republican nominee John McCain landed his own triple primary triumph.
However a tougher-than-expected showing from rival Mike Huckabee in Virginia reflected Mr McCain's struggle to close the deal with conservatives.
Mr Obama has now had eight consecutive wins and is expanding his lead in pledged convention delegates who select the party's nominee.
The wins for Mr Obama follow big weekend triumphs in Maine, Louisiana, Nebraska, Washington and the Virgin Islands.
In a count of nominating delegates, Mr Obama leads with 1,215 to Mrs Clinton's 1,185. A total of 2,025 delegates is needed for the nomination.
The role of some 440 still-undecided super-delegates is now likely to be critical.
All three of the contests occurred in fertile territory for Mr Obama, with large populations of the highly educated, high-income black voters who have favoured Mr Obama through this campaign.
But exit polls indicated Mr Obama cut dramatically into Mrs Clinton's core support groups.
A total of 168 Democratic delegates were at stake in the three states.
Mr Obama celebrated his wins at a rally in Wisconsin, which votes on 19 February and where he hopes to also win.
'We won the state of Maryland. We won the Commonwealth of Virginia. And though we won in Washington DC, this movement won't stop until there's change in Washington DC' he said.
Mr Obama barely referred to Mrs Clinton at all and instead turned his fire on Mr McCain in a preview of a potential November general election match up.
Conservatives wary of McCain
At the same time the McCain campaign was celebrating its victory in the so-called Potomac primary.
However John McCain's margin of victory was much closer than in the Democratic race.
Mr McCain is already looking forward to a general election match-up with the Democrats after the three wins, which will increase pressure on Mike Huckabee to give up his White House quest.
Despite Mr McCain's lead conservatives and evangelicals chose Mr Huckabee by a wide margin.
The 71-year-old senator fared better among those aged 65 or older. Veterans also showed a slight preference for Vietnam war veteran and former prisoner of war.
Mr McCain has a virtually insurmountable lead in the delegate count over Mr Huckabee with 812 to 217 in total delegates.
The winning Republican candidate needs 1,191 delegates to secure the nomination.
But Mr Huckabee captured two of three contests at the weekend as Mr McCain struggled to win over disgruntled conservatives unhappy with his record on immigration, taxes and other issues.
'Now my friends comes the hard part, and for America, the much bigger decision,' Mr McCain told supporters in Alexandria, Virginia.
'We do not know for certain who will have the honour of being the Democratic Party's nominee for president. But we know where either of their candidates will lead this country, and we dare not let them,' he said.
