With six days left before election day, US presidential front runner Barack Obama is making maximum efforts to close the deal as soon as possible, while underdog John McCain remains unyielding and aggressive against all the odds.

Obama, a Democratic senator from Illinois, when campaigning in the pivotal states of Ohio, Pennsylvania, Virginia and Florida, told his enthusiastic supporters they are "so close" to victory.

But McCain, a Republican senator from Arizona, was determined to fight to the end.

According to ABC World News, McCain is now focused on defending a number of Republican states, and has become more aggressive.

However, doubts about McCain's chances for the presidency are growing, even among fellow Republicans. Two Republicans once on his short-list for vice president sounded skeptical.

Mitt Romney, former governor of Massachusetts, hinted "the very real possibility of an Obama presidency." Tim Pawlenty gave a dour assessment of McCain's chances in his state, saying Obama has a pretty good advantage in his state of Minnesota.

OBAMA HOLDS ADVANTAGE IN ELECTORAL VOTES

The U.S. president is elected by the Electoral College, which is elected on a state-by-state basis and a person needs at least 270 of the total of 538 electoral votes to win the presidency.

Therefore, presidential candidates will focus on key states that will determine their fate. In this election, five states are considered very crucial.

The Republicans haven't lost Virginia in 44 years and they absolutely need Florida to reach 270 electoral votes.

Republicans never won the White House without carrying Ohio, and the last time Missouri backed a losing candidate was in 1956.

Although a traditional Democrat-leaning state, Pennsylvania is also pivotal because McCain does have a chance there.

However, new poll numbers for these states look grim for him.

A Survey USA automated poll of 648 likely Ohio voters shows Obama leading McCain 49 to 45 percent.

A Bloomberg poll of 639 likely Florida voters taken Oct. 25-27 shows Obama leading McCain 50 to 43 percent.

The Rasmussen Reports automated poll of 500 likely Pennsylvania voters taken Oct. 27 shows Obama leading McCain 53 to 46 percent.

Obama is also leading in Virginia and Missouri.

NATIONAL POLLS VARY A LOT

Although national polls continue to show Obama leading McCain, the size of Obama's lead, however, continues to vary widely in different surveys.

That's why McCain says the race is still close and the polls are inaccurate.

A Pew Research poll of 1,198 likely voters taken Oct. 23-26 shows Obama leading 53 to 38 percent.

The Gallup daily presidential tracking poll of 2,781 registered voters taken Oct. 25-27 has two likely voter models.

One model based on past voting behavior and current intention to vote shows Obama leading McCain 49 to 47 percent.

The second model based on current intention to vote shows Obama up 51 to 44 percent.

The Zogby daily presidential tracking poll of 1,203 likely voters taken Oct. 26-28 shows Obama leading by 5 percentage points.

The Rasmussen Reports daily presidential tracking poll for Oct.27 shows Obama leading McCain 51 to 46 percent.

The ABC News/Washington Post daily presidential tracking poll of 1,301 likely voters taken Oct. 24-27 shows Obama leading McCain 52 to 45 percent.

VOTERS PREFER OBAMA ON TAXES

The Wall Street Journal reports Republicans are losing ground in the battle over taxes -- turf they have dominated since the Reagan administration,

The latest Wall Street Journal/NBC News poll showed voters preferred Obama to McCain on taxes by 14 percentage points.

Obama will focus on the economy and how his tax plans would help the middle class in a half-hour political TV ad to be shown on major U.S. media networks.

The ad, called "Infomercial," will be broadcast on NBC, CBS, Fox, Univision, BET, MSNBC and TV One.