Washington, Sep 5 (IANS): US Republican presidential candidate Donald Trump was fighting back in polls and set to spend millions on TV advertisements as the US presidential election unofficially kicked off in force on Monday.

The Labor Day holiday marks the start of an intense period for campaigning with a little over two months left until the Nov 8 vote, The Telegraph reported.

Surveys showed a significant narrowing of the gap between Trump and his Democratic rival Hillary Clinton.

A Morning Consult poll released on Sunday showed Clinton up by two points, a lead that had been seven in the same poll three weeks ago.

The latest Reuters/Ipsos tracking poll of "likely voters" in all 50 states had the race as a statistical tie, registering an eight point swing against Clinton in the last week.

It also showed support for Trump among Republicans jumping six points over the past two weeks, giving him 78 per cent backing in his own party.

That was close to the 85 per cent Mitt Romney enjoyed as the Republican nominee in 2012.

An average of all recent polls showed Clinton's lead at 3.9 points, less than half what it was a month ago, The Telegraph added.

The boost she received after the Democratic Convention at the end of July has disappeared.

One poll conducted by the Los Angeles Times gave Trump a three-point lead nationally.

It came as Clinton faced a resurgence of criticism over her use of a private e-mail server while she was US Secretary of State, and the relationship between the Clinton Foundation and some of its donors.

However, Clinton maintained healthy leads in key battleground states including Pennsylvania, Ohio and North Carolina.

Trump finally began opening his funding war chest, buying $10 million in television advertisements.

So far, Clinton has outspent the billionaire by nine to one on advertising.