No, The Senate is Not a Worthwhile Check on the Prime Minister

Far better ones exist.  I really don't get where this idea comes from.  Can you name anything bad, ever that our Senate has prevented?  Did it prevent Trudeau's Martial Law?  Did it stop the internment of Japanese Canadians?  Racist laws like head taxes on Chinese immigrants?  Aboriginal residential schools?  The death penalty? 

You might say that the Senate needs reforms and election of its members to be more effective as a check on tyranny, but the US Senate has an even worse record of preventing injustice and atrocity, in fact spending much more time blocking laws to stop things like lynching and segregation than it did to block anything truly bad.  Even more recently the US Senate was more supportive than the US House of the Bush era abuses, from the Patriot Act, the Iraq War authorization, to the permanent detention without trial of many people at Gitmo. 

A Canadian elected Senate would still be controlled by the party leaders via our strict party discipline system. Elected Senators would have to stand for re-election as independents if they flout their party leader.  They would be relegated to back-bench opposition irrelevancy.  If the PM has a majority in the House, chances are s/he has a majority in the Senate (think how province-wide Senatorial elections would have turned out in 2011).  So even the hope that we have split government in each chamber will not tend to happen. In fact, if things are bad enough that a leader's own legislators are turning, House members are far more powerful in that they can trigger the Prime Minister's resignation by joining the opposition to defeat the government in a confidence vote.  Senators could block bills, but House members can force new elections.

The worst injustices perpetrated by governments tend to be popular in their time, which means politicians are a poor safeguard.  The proven, far more effective safeguard is the Courts, enforcing our Constitutionally embedded Charter of Rights.  Further, Canada, as a federal society leaves significant powers (including administration of justices and most criminal code enforcement) in the hands of the provinces.  Throw in Quebec being never more than 3 or 4 steps from independence (and Alberta not far behind it sometimes seems) and you really have a hard time concocting a viable "runaway Prime Minister" scenario with or without the Senate.

Finally, if by "tyranny" you literally fear Harper setting himself up as a literal dictator via some kind of "Enabling Act" - let's take a closer look at Hitler's rise.  When Weimar Germany's legislature passed the Enabling Act, Hitler had SA thugs all over the building, and had already banned a major opposition party, the Communists, so none of their members were even present to vote against it.  There's simply no basis for believing a Weimar Senate, if it had existed, would have not been equally coerced by intimidation and politics of the day to pass the thing.  Hitler had already seized defacto power.  The real lesson of dictatorships is that by the time the dictator gets around to formalizing powers in actual written laws, it's already too late for legislative opposition to stop him.  You can tell a similar story for the rise of Mussolini, whose rise to power begins by leading a fascist black-shirt army on a march to Rome.

Two thirds of the world's democracies do fine without upper houses.  Canada can too.  Meanwhile Senates often prove unable to prevent (or even contribute to) slides into dictatorship such as in Chile or Argentina.  Voters will have to remove Mr. Harper (assuming his own party doesn't do it first), and if they're waiting for the Senate to do something effective to block him, they'll be waiting a long time.