The Conservatives and the Opposition New Democrats have drawn their political battle lines in time for the return of MPs to Parliament after a two-month summer break.

Government House Leader Peter Van Loan picked up the attack on NDP Leader Tom Mulcair today in his first press conference of the fall sitting.

"In the last election, Thomas Mulcair campaigned on a platform that had right in it, in black and white, a $21 billion tax hike from carbon," Van Loan said.

Mulcair defended himself Sunday against Conservative claims the NDP wants to impose a carbon tax. "This one is called the big lie," said Mulcair in an interview with CBC's senior political correspondent Terry Milewski.

Van Loan is the latest to raise the issue, following statements by Prime Minister Stephen Harper and Conservative MPs that a vote for the NDP would be a vote for a carbon tax.

Harper said Friday that the NDP "is focused on a carbon tax and increasing taxes," speaking to reporters after making an announcement in Saint-Paul-de-l'Île-aux-Noix, Que.

But Mulcair is calling the claim a lie and the Conservatives "hypocrites" for telling it.

"It's very rare for me to use this word in politics, because you tend to try to rise above it, but that is a bald-faced lie by the Conservatives," said Mulcair.

"They know I've never proposed a carbon tax."

Conservatives, NDP proposed cap and trade in 2008

In fact, both the Conservatives and the NDP proposed a cap-and-trade system during the 2008 campaign — an idea the Tories have since abandoned.

"The Liberals had a very direct carbon tax and we thought that was regressive, we thought that a cap-and-trade system was the only way to reduce greenhouse gas emissions," said Mulcair.

House Leader Peter Van Loan outlines the Conservative government's fall agenda on Parliament Hill Monday morning. (Sean Kilpatrick/Canadian Press)

Mulcair is calling on the prime minister to take responsibility and put an end to false claims.

"There comes a time when Mr. Harper is going to have to assume some kind of morality in his discourse and that of his party and that of his MPs, and I'm calling him out on that. I'm asking Stephen Harper to stop lying, through his MPs, on the issue of whether or not the NDP has proposed a carbon tax," said Mulcair.

In June, the Conservatives launched their first attack ad against Mulcair warning of the NDP's "dangerous economic experiments including a carbon tax that'll raise the price of everything including gas, groceries, and hydro."

Partisan attacks aside, Parliament will resume with federal parties also drawing their battle lines over foreign takeovers and a second omnibus budget bill.

China's bid for Calgary's Nexen

With a federal review of China's National Offshore Oil Corp.'s (CNOOC) $15.1-billion bid to buy Calgary-based Nexen Inc. currently under way, the Conservatives are said to be leaning favourably towards the deal but Mulcair appears to be siding the other way.

Opposition House Leader Nathan Cullen discusses the NDP's plans for the fall sitting of the Commons, as Chief Opposition Whip Nycole Turmel looks on, on Parliament Hill Monday. (Sean Kilpatrick/Canadian Press)

"We're not dealing on an even playing field with Communist China. They have state resources, they don't accept the rule of courts, they don't follow environmental procedures," said Mulcair.

Stopping short of saying the Conservatives ought to reject the deal, the NDP leader said "why in heaven's name would we give up our own resources that way to another country?"

While Harper said the CNOOC bid did not come up during his meeting with China's President Hu Jintao during a bi-lateral meeting at the Asia Pacific summit in Russia last week, Harper did indicate he wants to open investment to China as long as it is willing to reciprocate.

The NDP and the Liberals have called into question the clarity of Canada's foreign investment rules.

The review, which began at the end of August, will take 45 days, but could be extended by 30 days or more.

2nd omnibus budget bill

The Conservatives have made it clear they will continue to focus on the economy and jobs with the next fiscal and economic update coming sometime in the fall.

But the worry for the opposition parties is that MPs will not be in a position to give a second omnibus budget bill the scrutiny it deserves as they say happened in the case of Bill C-38, which the government presented last March.

NDP House leader Nathan Cullen told CBC News there's nothing wrong with the concept of an omnibus budget bill per se.

"It depends on what's in the bill," Cullen said.

Deputy Liberal leader Ralph Goodale told CBC News while there are a lot of important issues, from education for First Nations to MPs' pensions, "you do not do justice to any one of those subjects if you lump them all together in some kind of toxic stew."

Green Party Leader Elizabeth May will be paying close attention to legislative changes Environment Minister Peter Kent is expected to make with respect to the Species At Risk Act, even if they are said to come outside the confines of a second omnibus budget bill.

May opposed major changes to environmental oversight included in the last budget bill.

With files from CBC's Terry Milewski