The standoff between the ruling Saenuri Party and the main opposition Democratic Party is deepening as the government’s controversial tax reform proposal has provided fresh ammunition for the opposition party to attack the ruling camp.

On Monday, the Democratic Party seized upon the unpopular tax plan to step up its assault on President Park Geun-hye. It kicked off a nationwide signature collection drive to thwart the reform plan, which was unveiled Thursday to levy more taxes on people in the middle and upper income brackets to finance Park’s campaign pledges.

The party’s rush followed an outdoor rally it held in central Seoul on Saturday to demand that Park offer an apology for the National Intelligence Service’s alleged meddling in the December presidential election and undertake a thorough reform of the spy agency.

The rally was the second of its kind organized by the party since it decided earlier this month to take to the streets to protest the ruling party’s attempt to stonewall the ongoing parliamentary investigation into the NIS.

Following the gathering, the party’s leaders and rank-and-file members participated in a massive candlelight vigil organized by civic groups to denounce the spy agency and ratchet up pressure on Park to reform it.

The party has pledged to hold protest rallies in other cities and continue to participate in the candlelight vigil, which is reminiscent of the mad cow nightmare five years ago.

The party’s focus on extraparliamentary activities suggests that it no longer pins much hope on the parliamentary probe into the NIS.

In fact, the probe is unlikely to shed much light on the intelligence agency’s alleged smear campaign against the Democratic Party’s presidential candidate in the run-up to the December election.

The success or failure of the probe depends to a large degree on whether legislators can make the two key witnesses ― former NIS director Won Sei-hoon and former Seoul police head Kim Yong-pan ― tell the truth.

But there is no way to force the two to testify in the scheduled parliamentary hearings because the relevant law allows witnesses to refuse to testify when such testimony may lead to criminal prosecution or conviction.

Won and Kim are highly likely to refuse to testify, given that they have already been indicted.

In this regard, the parliamentary probe has its limits in establishing the truth. But it was the opposition party that called into question the outcome of the prosecution’s investigation and insisted on parliamentary scrutiny.

Now, the opposition party is set to step up street protests, a tactic it used decades ago to fight against dictatorial governments. Street rallies proved effective again during the mad cow hysteria of 2008.

But the party should not be under any illusion that the arduous approach will work again. The tax reform proposal may energize its outdoor rallies to some degree, but definitely not to the same level as the mad cow disease protests did.

The party would be better off returning to parliament and focusing on reform of the NIS without taking aim at Park. This will make it look more trustworthy. The ruling party should open the way for the rival party’s return by taking a more open stance on NIS reform.