After an 18-month investigation, the RCMP has handed their file into Sen. Pamela Wallin’s questionable expense claims over to Crown prosecutors, CTV News has learned.

The possibility that charges could be imminent comes as Stephen Harper is seeking re-election to the country’s top office – and facing a campaign trail where the Senate expense scandal has already been a major topic of discussion.

While it is ultimately going to be up to the RCMP whether charges are laid, criminal defence lawyer Michael Spratt said it "wouldn’t be unusual to have the Crown review the brief to ensure there’s a reasonable prospect of conviction."

In court documents filed earlier this year, the Mounties allege Wallin fraudulently billed the Senate for more than $25,000 in expenses she incurred on trips to Toronto and Guelph, Ont., to “pursue her personal and business interests.”

None of the allegations have been proven in court and no charges have been laid.

If there are charges, however, that could further impact Harper, who previously defended Wallin in the House of Commons.

“I looked at the numbers,” the Prime Minister said on Feb. 13, 2013. “Her travel costs are comparable to any parliamentarian travelling from that particular area of the country.”

Former RCMP Commissioner Norman Inkster said the fact an election campaign is underway will not influence the timing of any charges that may be brought.

Meanwhile, emails released at the trial of Sen. Mike Duffy raise questions about whether the Prime Minister’s Office pressured Wallin into paying back some of her expenses claims ahead of an independent audit into Senate spending.

Harper’s former chief of staff Nigel Wright wrote a memo to former Conservative Senate Leader Marjory LeBreton, seemingly trying to protect Wallin.

“I would expect that Sen. Wallin would be given the opportunity to address any unusual expenses through repayment if that is appropriate, as she has already done with some inadvertently claimed,” an email dated Feb. 19, 2013 reads.

When asked by CTV News, Wallin’s lawyer Terrence O’Sullivan wouldn’t say whether Wallin was pressured by the PMO to reimburse the Senate.

“Because there are issues touched upon in the Duffy trial, it would be inappropriate for me to comment until that trial is over,” O’Sullivan said in a statement.

Wallin told a Toronto radio station last year that she had paid back some of her expenses, despite her belief that many of the travel claims in question were legitimate.

"I regret doing it," Wallin said at the time. “I would pay back what I think I owed, but not the large sum of the charges that were rung up because of retroactively imposed new rules.”

Wallin has repaid almost $150,000 in disallowed travel expenses.

Wallin, along with fellow Conservative-appointed senators Patrick Brazeau and Mike Duffy, was suspended from the Senate in November 2013 over inappropriate expense claims.

Liberal-appointed Sen. Mac Harb resigned from the Senate in August 2013 after paying back $231,000 of questionable housing and travel expenses.

Wright cut a personal cheque for $90,000 to cover Duffy's questionable living expenses, and that payment is now at the centre of Duffy’s ongoing criminal trial, currently adjourned until November.

Duffy faces 31 charges, including multiple counts of fraud and breach of trust, and has pleaded not guilty to all charges.

Brazeau and Harb have also been charged with defrauding the Senate by making false or misleading expense claims.

The senators' suspensions ended when parliament was dissolved, meaning they can once again work and collect their paycheques.

Wallin was at work Monday but did not offer any comment to CTV News.

With files from CTV Ottawa Bureau Chief Robert Fife