The Greater Toronto Area and some ridings in southwestern Ontario have high concentrations of voters who could make a big difference in the outcome of the federal election.

In Canada's largest city, three of the ridings now held by Liberals are facing stiff competition from the Conservatives.

Liberal incumbent and former cabinet minister, Joe Volpe, has been the MP for Eglinton-Lawrence since 1988. In the last election, he beat his Conservative challenger, Joe Oliver by less than 3,000 votes. With many more Tory blue signs visible on the riding's streets than in previous elections, Oliver is mounting a strong campaign partially buoyed by support from a large Jewish population that has increasingly supported the Harper government's pro-Israeli stand.

Don Valley West is another urban riding where the Liberals won over the Conservatives by less than 3,000 votes in the last election. This constituency has the highest average income in Canada, but a mix of high-and low-income neighbourhoods. In 2008, first-time Liberal candidate, Rob Oliphant, held on to the riding for his party, after high-profile MP and cabinet minister John Godfrey retired. The current Conservative, businessman John Carmichael is running for the third time and hopes to overtake the Grits.

The riding of Willowdale encompasses part of the former city of North York, north of highway 401. Lawyer Martha Hall Findlay, a contender for the Liberal leadership in 2006, was first elected in an early 2008 byelection when the seat was vacated by Jim Peterson, who retired. Later that year, Findlay was returned as MP in the general election. Her Conservative opponent for the May 2 vote is businessman Chungsen Leung, who seeks a first win. The riding comprises large Chinese, Korean, Iranian and Jewish communities.

Some of the races in the 905 suburbs could be close. One is the riding of Vaughan, with its Italian enclave of Woodbridge north of the city, where Conservative and one-time OPP Commissioner Julian Fantino first won in a November 2010 byelection. The former Toronto police chief is in a fight to keep his seat against the Liberal hopeful, former local and regional councillor, Mario Ferri.

Another battleground is Brampton-Springdale to the west, where Liberal Ruby Dhalla won in 2008 by a narrow margin against Conservative challenger Parm Gill. This year, attempting to put 2009 allegations of mistreating two domestic workers behind her, she faces off against him again. And to the east in Ajax-Pickering, Conservative Chris Alexander will try to unseat three-term Liberal MP, Mark Holland, who edged out his Tory opponent, Rick Johnson, by just over 3,000 votes in the last election.

In southwestern Ontario, Kitchener Centre, Kitchener-Waterloo and Guelph are notable ridings with more Liberal-Conservative contests that could go either way, depending on the mood of the voters. Southwestern Ontario used to be a bastion of Liberal support, but this year a couple of interesting rematches are taking place.

In Kitchener Centre, former Liberal MP, Karen Redman, narrowly lost in 2008 to Conservative Stephen Woodworth, a legal advisor. Redman is trying to make a comeback this time. The result two years ago was similar next door in the riding of Kitchener-Waterloo, home to two universities and technology companies like Research in Motion. In the last federal election, Andrew Telegdi, long-time former Liberal MP, lost the seat by only 17 votes to Peter Braid, the current incumbent Conservative, a communications consultant. Telegdi is running to take back Kitchener-Waterloo on May 2.

The riding of Guelph will be one to watch with the Conservatives fielding first-time candidate, Marty Burke, an Air Canada pilot and former UN peacekeeper in the Middle East. Burke is trying to grab the riding from the Liberal incumbent, Frank Valeriote, who won over his Conservative opponent, Gloria Kovach in 2008 by less than two-thousand votes.

Dr. Peter Woolestencroft, political scientist and professor emeritus at the University of Waterloo, was asked by CTV News about the top issue in this election. He said, "There are lots of things the parties are talking about, but I don't think people are talking about much. In the end, I think it'll be the question ‘who do you want to have as Prime Minister,' and I think that, for most people, will be the deciding factor."