The 144,000 are a remnant of the 12 tribes, they do not have any mission or power in the time they are sealed. The only ones that have power in the 3 1/2 years before their return are the two witnesses.

Re:12:17:
And the dragon was wroth with the woman,
and went to make war with the remnant of her seed,
which keep the commandments of God,
and have the testimony of Jesus Christ.

Re:11:3-7:
And I will give power unto my two witnesses,
and they shall prophesy a thousand two hundred and threescore days,
clothed in sackcloth.
Re:11:4:
These are the two olive trees,
and the two candlesticks standing before the God of the earth.
Re:11:5:
And if any man will hurt them,
fire proceedeth out of their mouth,
and devoureth their enemies:
and if any man will hurt them,
he must in this manner be killed.
Re:11:6:
These have power to shut heaven,
that it rain not in the days of their prophecy:
and have power over waters to turn them to blood,
and to smite the earth with all plagues,
as often as they will.
Re:11:7:
And when they shall have finished their testimony,
the beast that ascendeth out of the bottomless pit shall make war against them,
and shall overcome them,
and kill them.

The 144,000 are the 'small in number' from the verse below and if they were not protected in that time the dragon is hunting people they also would die before the return.

Eze:39:23:
And the heathen shall know that the house of Israel went into captivity for their iniquity:
because they trespassed against me,
therefore hid I my face from them,
and gave them into the hand of their enemies:
so fell they all by the sword.

De:4:27-31:
And the LORD shall scatter you among the nations,
and ye shall be left few in number among the heathen,
whither the LORD shall lead you.
And there ye shall serve gods,
the work of men's hands,
wood and stone,
which neither see,
nor hear,
nor eat,
nor smell.
But if from thence thou shalt seek the LORD thy God,
thou shalt find him,
if thou seek him with all thy heart and with all thy soul.
When thou art in tribulation,
and all these things are come upon thee,
even in the latter days,
if thou turn to the LORD thy God,
and shalt be obedient unto his voice;
For the LORD thy God is a merciful God;)
he will not forsake thee,
neither destroy thee,
nor forget the covenant of thy fathers which he sware unto them.